1. Field of the Invention
The present invention relates to a high current ion implanter and method of ion implant by the implanter, and more specifically relates to a high current ion implanter to be used to perform ion implantation of 1.times.10.sup.14 (1E14) atms/cm.sup.2 or more using a beam of large current (1 mA or more) in a semiconductor substrate (wafer) in a semiconductor process, and method of ion implant by the implanter.
2. Description of the Related Art
A high current ion implanter is used in cases where ion implantation processing is performed continuously, for example, to plural sheets of wafer using a beam current of mA order or more, and such ion implantation apparatus is generally called a high current implanter.
A high current ion implanter is used, for example, to form an ion implantation layer of a source/drain region in a MOS transistor manufacturing process or to form a high density laver of arsenic (As) at the substrate side serving as an electrode part of a trench capacitor. Such layer formation is performed in an ion implantation process requiring a high ion implantation amount (dose amount) of about 1E15 atms/cm.sup.2 or more, and it seems advantageous to raise an ion beam current as large as possible from the perspective of mass production efficiency (throughput).
When an ion implantation layer is formed using a high current ion implanter, for example, in a source/drain region in a MOS transistor manufacturing process where a device is already formed on a wafer substrate, it is known that as the ion beam current is raised charge-up occurs and serious damage is produced in the device subjected to ion implantation. In order to suppress the charge-up, a high current ion implanter is provided with a mechanism called an electron shower. That is, if an ion beam irradiates a surface of a sample (wafer) when a device formed on the substrate has a structure not grounded electrically, since ions of positive charge collide with the wafer surface, the wafer is electrified and positively charged. To prevent this, the electron shower is an inevitable function for an ion implantation apparatus.
The electron shower which prevents or suppresses charge-up as above described operates by an electron beam being irradiated together with the ion beam to the sample surface thereby neutralizing the plus charge. The electron shower is performed in an ion implantation apparatus as shown in FIG. 4 where a secondary electron implantation cylinder 4 with a tungsten filament 3 and a grounded extension cylinder 11 are installed at the downstream side of a bias plate 2 immediately before the ion beam B enters an implantation chamber 1. More concretely, current flows through the tungsten filament 3 thereby emitting thermoelectrons, the thermoelectrons are irradiated to a metal surface within the secondary electron implantation cylinder 4 thereby emitting secondary electrons, and the secondary electrons are applied to the wafer surface irradiated by the ion beam B thereby neutralizing the plus charge.
Usually in a process where there is fear of damage due to the charge-up as above described, a manufacturing engineer first supplies electrons to a wafer in a TEG (test element group) subjected to ion implantation (ion beam current) via an electron shower or plasma corresponding to the ion beam current. The engineer optimizes the number of secondary electrons emitted from the mechanism to prevent the charge-up, and sets the ion implantation conditions using an ion beam current as large as possible considering the desired productivity.
The above description describes the damage caused by the charge-up occurring while the ion beam current is raised and ions are emitted. In cases where a high density layer of n+ at the substrate side serving as an electrode part of a trench capacitor of a DRAM (dynamic RAM) is formed by ion implantation, even if charge-up occurs on the wafer, no thin oxide film susceptible to damage from charge-up exists on the side of the wafer exposed to the ion beam. It has therefore been generally thought that the ion beam current could be increased without limit during ion implantation.
According to experiments by the present inventors, however, regarding ion implantation to the trench capacitor of a DRAM, it has been confirmed that under conditions where the ion beam current is constant, even if the number of secondary electrons generated from the electron shower is set to vary largely, the electric characteristics (charge holding characteristics) of the capacitor and the yield of the device do not vary. On the other hand, it has been also confirmed that as the ion beam current is increased, both the electric characteristics (charge holding characteristics) of the capacitor and the yield of the device deteriorate.
As described above, when an ion implantation layer is formed on the source/drain region where a device has been previously formed on the wafer substrate, if the ion beam current is increased, the charge-up generated on the wafer produces a problem by breaking the insulation film of the device. It has been found out that if the ion beam current is increased in such a manner, deterioration of the device occurs which can not be explained from the charge-up even when a high density layer of n+ at the substrate side serving as an electrode part of the trench capacitor is formed by ion implantation.
Therefore, the present inventors have repeated the study described above in order to clarify the mechanism of deterioration of the device element or its main cause which can not be explained by charge-up phenomenon. That is, an object of the present invention is not only to eliminate the deterioration of the device characteristics in the ion implantation process for the trench capacitor of a DRAM, but also to provide a high current ion implanter and method of ion implant by the implanter which can increase the beam current without deteriorating the device characteristics.